In many large scale file systems, data is replicated from one node to one or more other nodes. Storing multiple copies of the same data helps keep the data safe as well as facilitates faster read access to the data. By storing multiple copies of the same data, if the storage mechanism used to store one copy becomes unavailable, the data may still be available from a different storage mechanism.
Storage mechanisms for data may be located in different physical locations. An example of a storage mechanism for data is a hard-disk drive. A group of hard-disk drives may be stored in one location, such as in a cabinet. To ensure the data is available if a problem is encountered at any single location, data may be replicated to a different physical location.
Whenever a user wishes to retrieve a copy of data from a large scale file system, the user may issue a request for the data to the large scale file system. In response, the file system may route the request so that the request is serviced by the storage mechanism in the closest physical location to the origination of the request. Routing the request in this manner is done to both minimize the latency for retrieving the data as well as minimize the network bandwidth involved in processing the request.
Data may be stored in a variety of different persistent storage mediums, such as a hard-disk drive. A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When a HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head (hereinafter “head”), which is positioned over a specific location of a disk by an actuator.
A head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a head and the surface of a magnetic-recording disk must be tightly controlled. An actuator relies on a suspension's force on the head to provide the proper distance between the head and the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A head therefore is said to “fly” over the surface of the magnetic-recording disk.